Acutally, Doctor Who (Tom Baker) and his computer-dog K-9 (voiced by
John Leeson) wanted to go on an extended holiday somewhere in the sun, but
instead, the all powerful White Guardian (Cyril Luckham) persuades
him to go looking for the Key to Time, an all powerful intrument to
restore order int he universe ... but since it's all powerful, the Key to
Time (actually a cube) was broken into six pieces, scattered throughout
the universe and disguised at whatever it is to not arouse any suspicion.
And the Guardian even gives the protesting Doctor a new companion, pretty
and intelligent but hopelessly naive Romana (Mary Tamm).

The search for the first segment of the key takes the Doctor and
company to a medieval-like world, where a very likeable conman from earth,
Garon (Iain Cuthbertson), and his young assistant Unstoff (Nigel
Plaskitt) try to sell the whole planet - which they of course do not
own - to the powermad and violent exiled monarch Graff Vynda-K (Paul Seed)
by giving him the impression that the planet is rich in an all-powerful
mineral, Jedrich ... but the only piece of Jedrich on the planet was
actually brought by Garron himself to make his story more believable ...
and that piece of Jedrich is of course a segment of the Key to Time.

Soon enough, Garron and Unstoff have the Graff's money and their piece
of Jedrich, but somehow the Graff has found out, and all of a sudden not
only is Garron arrested, but the Doctor and Romana as well, who just tried
to get the Jedrich from Garron. And Unstoff, who has the Jedrich and the
money, hides in the catacombs under the city, where soon enough the city's
head guard (Prentis Hancock), the Graff and his men, and the Doctor,
Romana and Garron, who have been freed by K-9, go looking for him. Too bad
though that down in the catacombs there are dragons.

In the end, all of the Graff's men are killed by a cave-in and he
himself is killed by a timebomb he tried to plant on the Doctor, the
Doctor, K-9 and Romana however, as well as Garron and Unstoff, get out of
it all in one piece - and the Doctor even manages to keep Garron from
stealing the Jedrich from him ...

A well-written and wonderfully, if hammy, acted blend of sci-fi and
historical drama, filmed in some nice if not always terribly convincing
sets (left over from an adaptation of Anna Karenina) make this a
nice first episode of the Key to Time-storyarc. Totally
likeable.